We will measure resting energy expenditure and total energy expenditure in 10-12 month old infants and 7- 8 year old children with cystic fibrosis. These two groups have very different physical activity patterns. Infants are not yet ambulatory, so if their resting energy expenditures are higher than those of healthy infants, they are unable to compensate by lowering their physical activity. Therefore, their actual total energy needs may be much higher that those of healthy infants. In contrast, children may compensate for higher resting energy expenditure by decreased physical activity. However, the extent of this compensation is unknown. Knowledge of the total energy expenditures of these patients is necessary to accurately gauge their energy requirements. Our long-term objective is to gain insight into the total energy needs of CF patients in order to provide appropriate nutritional support. The information obtained in this project will provide us with a valuable foundation from which to design future dietary therapies. While early and appropriate dietary intervention is important in CF patients of any age, it is crucial in infants and children who are still growing and developing.